This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A246718 #22 Apr 04 2015 01:35:51
%S A246718 2,2,3,44,128,619,4134,28628,229132,2107538,21438790,238754555
%N A246718 a(n) is the number of different ways of concatenating the numbers {3^k, k=0,...,n} so as to produce a prime number.
%C A246718 A PARI program distinct from that below was used to compute a(14) using four windows in under a month, but the value was lost.
%C A246718 It is neither trivial nor very difficult to establish that distinct permutations lead to distinct values.
%e A246718 The a(1)+a(2)+a(3)+a(4)=51 primes corresponding to the first four terms are, in increasing order, 13, 31, 139, 193, 12739, 19273, 32719, 1273981, 1278139, 1279813, 1381279, 1398127, 1812793, 1819273, 1927813, 2713981, 2718139, 2718193, 2731819, 2738119, 2738191, 2739181, 2781139, 2781193, 2781913, 2793181, 2793811, 2798113, 3127819, 3127981, 3192781, 3271981, 3279811, 3811279, 3812719, 3812791, 3912781, 3918127, 8113279, 8113927, 8119273, 8127319, 8131927, 8139127, 8193127, 9127813, 9181327, 9273181, 9327181, 9812731 and 9813127. Concatenations not shown, such as 931 = 7^2 * 19 and 1392781 = 13 * 107137, are all composite.
%o A246718 (PARI) a(n,v=vector(n+1,k,Str(3^(k-1))))=sum(k=1,(n+1)!,ispseudoprime(eval(concat(vecextract(v,numtoperm(n+1,k)))))) \\ _M. F. Hasler_, Jan 13 2015
%K A246718 base,nonn
%O A246718 1,1
%A A246718 _James G. Merickel_, Nov 15 2014
%E A246718 Edited and verified up to n=9 by _M. F. Hasler_, Jan 13 2015